1. Field of the Invention
This invention relates to encapsulation of water dispersed materials. It more particularly relates to UV curable microencapsulated systems.
2. Description of the Related Art
Processes of microencapsulation are well known. U.S. Pat. Nos. 2,730,456, 2,800,457; and 2,800,458 describe methods for capsule formation. Other useful methods for microcapsule manufacture are: U.S. Pat. Nos. 4,001,140; 4,081,376 and 4,089,802 describing a reaction between urea and formaldehyde; U.S. Pat. No. 4,100,103 describing reaction between melamine and formaldehyde; and British Pat. No. 2,062,570 describing a process for producing microcapsules having walls produced by polymerization of melamine and formaldehyde in the presence of a styrenesulfonic acid. Microcapsules are also taught in U.S. Pat. Nos. 2,730,457 and 4,197,346. Forming microcapsules from urea-formaldehyde resin and/or melamine formaldehyde resin is disclosed in U.S. Pat. Nos. 4,001,140; 4,081,376, 4,089,802; 4,100,103; 4,105,823; 4,444,699. Alkyl acrylate—acrylic acid copolymer capsules are taught in U.S. Pat. No. 4,552,811. Each patent described and cited in this specification is incorporated herein by reference to the extent each provides guidance regarding microencapsulation processes and materials.
U.S. Pat. No. 4,622,267 describes an interfacial polymerization process wherein a microcapsule wall of a polyamide, an epoxy resin, a polyurethane, a polyurea or the like is formed at an interface between two phases. The core material is initially dissolved in a solvent and an aliphatic diisocyanate soluble in the solvent mixture is added. Subsequently, a nonsolvent for the aliphatic diisocyanate is added until the turbidity point is just barely reached. This organic phase is then emulsified in an aqueous solution, and a reactive amine is added to the aqueous phase. The amine diffuses to the interface, where it reacts with the diisocyanate to form polymeric polyurethane shells. A similar technique, used to encapsulate salts which are sparingly soluble in water in polyurethane shells, is disclosed in U.S. Pat. No. 4,547,429. U.S. Pat. No. 3,516,941 teaches polymerization reactions in which the material to be encapsulated, or core material, is dissolved in an organic, hydrophobic oil phase which is dispersed in an aqueous phase. The aqueous phase has dissolved materials forming aminoplast resin which upon polymerization form the wall of the microcapsule. A dispersion of fine oil droplets is prepared using high shear agitation. Addition of an acid catalyst initiates the polycondensation forming the aminoplast resin within the aqueous phase, resulting in the formation of an aminoplast polymer which is insoluble in both phases.
Common microencapsulation processes can be viewed as a series of steps. First, the core material which is to be encapsulated is emulsified or dispersed in a suitable dispersion medium. This medium is preferably aqueous but involves the formation of a polymer rich phase. Most frequently, this medium is a solution of the intended capsule wall material. The solvent characteristics of the medium are changed such as to cause phase separation of the wall material. The wall material is thereby contained in a liquid phase which is also dispersed in the same medium as the intended capsule core material. The liquid wall material phase deposits itself as a continuous coating about the dispersed droplets of the internal phase or capsule core material. The wall material is then solidified.
The above processes typically involve formation of an oil in water (o/w) emulsion. An aqueous suspension of microcapsules or coating of microcapsules is thereby typically obtained. Such capsule systems have the drawback of not readily encapsulating aqueous materials. A further drawback is that the resulting coatings as oil in water emulsions must be dried extensively. Carbonless coatings for example are applied on coating machines having substantial dryer sections for purposes of driving off the excess moisture.
Oil containing microcapsules dispersed in water are the largest volume use of microcapsules today and typically used for carbonless paper coatings. Microcapsules are used in pharmaceuticals, pesticides, paints, adhesives, sealants, inks, in addition to uses in carbonless or thermal paper applications.
Uses of microcapsules in adhesive is exemplified by Baetzold et al. (U.S. Pat. No. 6,084,010) which teaches solid, tacky or non-tacky hot melt glue compositions having incorporated therein microcapsules of a solvent capable of softening or further tackifying the hot melt.
Another type of encapsulated adhesive and sealant is that where the adhesive or sealant material or, in the case of a curable adhesive or sealant material, the components thereof are encapsulated in a single capsule. These capsules are typically applied to a substrate in a binder system that is non-tacky and dry to the touch. In this way, otherwise tacky or liquid flowable adhesives can be pre-applied, but not activated or bond forming until the capsule walls themselves are fractured releasing or exposing the adhesive materials. For example, Eichel (U.S. Pat. No. 2,986,477) teaches the encapsulation of tacky adhesive materials. Wallace (U.S. Pat. No. 4,428,982) teaches the encapsulation of curable anaerobic adhesives wherein the encapsulating material is air permeable so that the curable adhesive remains in a liquid or uncured state in the capsule until use. Schwantes (U.S. Pat. No. 6,592,990) teaches encapsulated adhesives, particularly pressure sensitive adhesives, wherein the adhesive is formed in-situ, after encapsulation of the ingredients therefore.
A common feature of each of the above systems is use of microcapsules encapsulating organic solvents. These typically are formed from oil in water emulsions, and usually require a substrate drying step.
The present invention by contrast relates to encapsulation of water dispersed materials, the microcapsule internal phase comprising an aqueous solution of a water soluble or water dispersible core material.
It would be advantageous and an advance in the art if a microcapsule suspension could be prepared that does not require extensive drying but could be formed as a curable coating. Carbonless paper or adhesive coated substrates or perfumed magazine inserts, or other coated substrates where capsules having a water soluble or water dispersible core are useful, when made with such a slurry or suspension would obviate needs for extensive drying of the substrate.
It is an object of the present invention to teach a novel coating containing microcapsules which is curable with actinic radiation and does not require substantial driving off of excess moisture.